The Collaborative International Dictionary of English v.0.48:
Unanswerable \Un*an"swer*a*ble\, a. Not answerable; irrefutable; conclusive; decisive; as, he gave an unanswerable argument. -- Un*an"swer*a*ble*ness, n. -- Un*an"swer*a*bly, adv. [1913 Webster]